Device for automatically terminating x-ray exposures



J. FRANSEN 2,82 3 DEVICE FOR AUTOMATICALLY TERMINATING X-RAY EXPOSURES April 1, 1958 Filed June 15, 1954 INVENTOR JACOBUS FRANSEN %W/ AGENT United States Patent DEVICE FOR AUTOMATICALLY TERMINATING X-RAY EXPOSURES Jacobus Fransen, Eindhoven, Netherlands, assignor, by

mesne assignments, to North American Philips Company, Inc., New York, N. Y., a corporation of Delaware Application June 15, 1954, Serial No. 436,904

Claims priority, application Netherlands June 16, 1953 4 Claims. (Cl. 250-95) The present invention relates to X-ray apparatus.

In X-ray apparatus for diagnosis it is known to employ a device by means of which an exposure is terminated automatically after the X-ray tube has produced suflicient radiation to obtain the required blackening of the photographic plate or film. The means employed for this purpose convert the X-rays into an electric current which is utilized to operate a switching mechanism for interrupting the load of the tube.

The electric current is produced by means of a discharge vessel or'other electronic cell which may, for example comprise an ionization chamber sensitive to X-rays. Alternatively, the X-rays may be converted by means of a fluorescent screen into light rays which fall on a photoelectric cell.

The conventional devices sufler from the disadvantage that the load of the X-ray tube is not always interrupted after the correct time interval, which may be explained as follows. The current intensity supplied by the electronic cell is proportional to the average intensity of the X-rays incident on the cell. In the case of the X-rays being littleweakened in parts of the exposed object, so that the average intensity exceeds that of the blackened area interesting for the diagnosis, the exposure will be terminated after a time insuflicient for obtaining an adequate blackening of the photographic plate or film, hence for obtaining a satisfactory exposure. Alternatively, the electronic cell may incidentally be struck by greatly weakened radiations so that the exposure period will exceed the optimum value.. .Advantageously the discharge vessel or electronic cell should only be struck by that part of the X-rays which produces the blackening in the area which is of importance for the diagnosis.

A correct positioning requires a certain amount of skill and is sometimes even impossible, since in making an exposure it cannot always be ascertained with certainty what part of the exposure is of importance for the diagnosis, In order to mitigate this disadvantage use has been made of a discharge vessel with a number of discharge paths between surfaces of difierent form, which paths are employed eachindividually or in combination dependent upon the presumed size of the area concerned.

The present invention relates to devices of the aforesaid type and has for its purpose, regardless of the kind of exposure to .be made by means of X-ray apparatus, to bring the actual loading period as closely as possible into agreement with that yielding the optimum results. In accordance with theinvention, the device comprises a number of discharge vessels which, in the case. of direct irradiation with X-rays or through the intermediary of a source of secondary rays, produce electric currents, the individual electric currents being converted by means of capacitors into potential ,diiferences which vary with the time and are operative in the grid circuits of discharge tubes. whereof the common anode currents are supplied to the switching mechanism in order to interrupt the load of the X ray tube.

The production of an electric current by a discharge 2,829,273 Patented Apr. 1, 1958 vessel or electronic cell may result from conductivity vari ations of a gas filling due to ionization or conductivity variations of photo-conductive material under the action of irradiation in a closed space containing two electrodes and whereof the discharge path forms part of an outer circuit associated with the electrodes so that each electrode is connected to a terminal of a voltage supply. In another known form of said cells suiting the purpose, use is made of an intermediate material having photoelectric properties and in which electrical energy is produced by irradiation.

The charging voltage of the capacitors may serve as a grid potential of the discharge tubes or be deducted from a fixed potential applied to the grids. In the first-- mentioned case, the increasing potential ditference across the terminals of the capacitors during the exposure involves a decrease of the grid potentials, hence a decrease in anode current. In the other case a negative bias is used and the capacitor voltages counteract said potential.

so that the control voltages constantly decrease and the discharge currents constantly increase during the loading period.

Each cell contributes only partly to the current variation required from the beginning of the exposure to operate the switching mechanism. Mutual diiferences of grid voltage across the control electrode of such a discharge tube the slope of said curve decreases, a given voltage variation at a lower grid voltage has a greater influence on anode current variations than it has in an. area of higher grid voltage wherein the mutual conduct-' ance is lower. Consequently, a voltage variation pro-.- portional to the time across a capacitor included in the,

grid circuit will relatively produce smaller anode current variations if said variation is large than if it is small.

It. therefore follows that the influence of exposure in-v tensities, exceeding the average value required for the X-ray exposure, on the total exposure period is less. This effect can be increased by choosing the ditferent voltages to be such that the voltage variations in the grid circuits, caused by the charging voltages of the capacitors, exceed the grid space of the tubes.

Inasmuch as the voltage increase across the capacitor exceeds said particular grid voltage this increase no longer affects the value of the anode current.

Upon an increase in grid potential of the discharge tubes as a result of an increase in charging voltage of the capacitors over a larger range flian the applied negative grid voltage, so that the anode currents increase initially,

the increase discontinues, in this case, due to the production of grid current in the discharge tubes. the grid potential does not exceed the cathode potential, since on approachingthis potential the grid currents carry off the charge still supplied to the capacitors, without any further variation of the capacitor voltages. I

With most capacitors the voltagevariations will attain different values in a given time, hence each discharge tube contributes toa difierent degree tothe current operating the switching mechanism. If said current exceeds a predetermined valueit operates said mechanism, so that the instant at which this occurs isdetermined by the discharge currents of those'tubes having capacitors included in their grid circuitsstill'undergoing potential variations F which affectthe anode currents. In' this case, also, the influence of the little weakened radiation on the exposure In this case,

period is greatly reduced so that the actual exposure time is more in agreement with that which is required for obtaining the optimum results.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, wherein the single figure is a schematic diagram of an embodiment of the X-ray apparatus of the present invention. In the figure, a number of electronic cells is denoted by reference numeral 1. The cells 1 are placed behind a luminescent screen 2 which is struck by X-rays from an X-ray tube 3 after having traversed the object 4 and the photographic plate or film 5.

. The cells 1 are connected through capacitors 7 to a voltage supply 6. Each capacitor 7 forms part of the grid circuit of a discharge tube 8. Each discharge tube 8 comprises a cathode 9 which is connected through a source of positive voltage 10 to one terminal of the capacitor 7 and a control electrode 11 which is directly connected to the other terminal of the capacitor 7. The anodell of each tube 8 is connected to those of the other tubes 8, hence said tubes are connected in parallel in a circuit which comprises an energizing coil 13 of an electromagnetic relay 14. The anode currents are supplied by a common voltage source 15.

Prior to the device becoming operative, a switch 16 connected in parallel with the supply source 10 is opened for a short time. Forthispurpose, a press-button switch may be employed whereof the contacts are closed it no pressure is exerted. Since the supply source 10 is shortcircuited when the switch 16 is closed, a resistor 17 limitiug the short-circuiting current is connected in series with said switch.

With the switch 16 closed, the capacitors 7 are connected with one of the terminals to the cathodes 9 of the discharge tubes 8. Any charge driving the grid-connected terminals of the capacitors 7 positive relatively to the cathodes 9 may flow otf via the grids 11 of said discharge tubes. The grids are at cathode potential, at whichthe discharge tubes 8 deliver the full anode current and thearmature 14 is attracted by the relay coil 13 so that the switch 18is open.

With the switch 16 open, the capacitors 7 are charged due to the polarity of the supply source 10, without altering the condition of the switching mechanism comprising the relay coil 13, the armature 14 and the switch 18. The exposure is started by closing the switch 16 so that the grids l1 are driven negative and the discharge tubes 8 do not pass current. As a result the relay coil 13 releases the armature 14 and closes the switch 18 so that the circuit of the supply device 19 for the X-ray tube 3 is connected to the terminals of the source of power supply 20.

A part of the X-rays supplied by the X-ray tube 3 reaches luminescent screen 2- after having traversed the object 4 and the. photographic plate or film and is converted into luminescence in said screen, the intensity distribution corresponding to that of the X-rays incident on the sensitized material used for making the exposure. Each light-sensitive cell 1 is struck by a part of the luminescence and becomes conductive to a greater or lesser degree according as the average intensity of the radiation incident on the cells 1 is higher or lower. Due to the terminal voltage fromtthe supply source 6, currents are produced in the circuits, which currents change the voltage variations across each ofthe capacitors 7. The grid potentials ofdischarge tubes 8 are thus gradually driven less negative and theanode currents of the said discharge tubes. are increased.

The maxinium contribution:ot each discharge tube 8 totthc current throughthe relay coil 13 is limited to the maximumanode cur-renteach said tube is able to supply if the grid 11 attains substantially the same potential as the cathode, 9. Since, consequently, the contributionof each discharge tube 8 to the:totalcurrentislimitedto a maximum value, the influence of an exceedingly strong radiation incident on one or a few of the total number of cells 1, which results in the current in a discharge tube associated with such a cell increasing to the maximum value in a time shorter than the exposure time, only.

slightly affects the actual time after which the switching mechanism is operated.

It may happen that none of the cells is struck by an exceedingly strong radiation, and an average intensity of a highly increased. value occurs in none of the cells 1. In this case, a favorable property of the anode-current versus grid-voltage characteristic of the discharge tubes 8 is utilized to counteract the tendency of lightly conducting parts of the object 4 to prolong the loading period of the X-ray tube. As a result of the slope of said charac teristic increasing with a decrease in grid potential, the influence of under-exposure of the plate 5 on the loading period of the X-ray tube is lessened. Thus, despite considerable local deviations from the average radiation intensity, which is considered advantageous for an exposure of the plate 5, there is no considerable deviation of the exposure period from the optimum exposure time corresponding to said intensity.

Modifications of the example given, with regard to the polarity of the capacitors and their charging condition at the beginning of the exposure, are possible without fundamentally affecting the operation of the device.

While the invention has been described by means of a specific example and in a specific embodiment, I do not wish to be limited thereto, for obvious modifications will occur to those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. In X-ray apparatus, a device comprising an X-ray tube for producing X-rays, means for supplying energy to said tube, means including a switching mechanism for interrupting the supply of energy to said tube, means comprising a plurality of X-radiation responsive devices for producing a plurality of electric currents each having an intensity as determined by the intensity of X-radiation impinging on the said responsive devices respectively, means including a plurality of capacitors for converting each of said electric currents into a potential difference, a plurality of discharge tubes each having a grid and an anode, means for applying said potential difference to the respective grids of said discharge tubes, and means for simultaneously supplying currents from the anodes of said discharge tubes to said switching mechanism thereby to automatically interrupt the supply of energy to said X-ray tube at a predetermined value of the combined anode currents.

2. In X-ray apparatus, a device comprising an X-ray tube for producing -X-rays, means for supplying energy to said tube, means including a switching mechanism for interrupting the supply of energy to said tube, means comprising a plurality of X-radiation responsive devices for producing a plurality of electric currents each having an intensity as determined by the intensity of X-radiation impinging on the said responsive devices respectively, means including a plurality of capacitors for converting each of said electric currents into a potential difference, said X-radiation responsive devices comprising cells for supplying currents to said capacitors, a plurality of discharge tubes each having a grid and an anode, means for applying said potential diiference to the respective grids of said discharge tubes, said means including a plurality of capacitors producing an increase in potential at the grid of each of said plurality of discharge tubes due to the presence of said electric currents, and means for simultaneously supplying currents from the anodes of said discharge tubes to said switching mechanism thereby to automatically interrupt the supply of energy to said X-ray tube at a predetermined value of the combined anode currents.

3. In X-ray apparatus, a device comprising anX ray tube for producing ,X-rays, means for supplying energy to said tube, means including a switching mechanism for interrupting the supply of energy to said tube, means comprising a plurality of X-radiatiou responsive devices for producing a plurality of electric currents each having an intensity as determined by the intensity of X-radiation impinging on the said responsive devices respectively, means including a plurality of capacitors for converting each of said electric currents into a potential difference, said X-radiation responsive devices comprising cells for supplying currents to said capacitors, a plurality of discharge tubes each having a grid and an anode, means for applying said potential difference to the respective grids of said discharge tubes, said means including a plurality of capacitors producing a decrease in potential at the grid of each of said plurality of discharge tubes due to the presence of said electric currents, and means for simultaneously supplying currents from the anodes of said discharge tubes to said switching mechanism thereby to automatically interrupt the supply of energy to said X-ray tube at a predetermined value of the combined anode currents.

4. In X-ray apparatus, a device comprising an X-ray tube for producing X-rays, means for supplying energy to said tube, means including a switching mechanism for interrupting the supply of energy to said tube, means comprising a plurality of X-radiation responsive devices for producing a plurality of electric currents each having an intensity of X-radiation impinging on the said responsive devices respectively, means including a plurality of capacitors for converting eachof said electric currents into a potential diiference, a plurality of discharge tubes each having a grid and an anode, means for applying said potential difference to the respective grids of said discharge tubes, means for applying a potential of predetermined value to said respective grids of said discharge tubes, said predetermined value being determined to provide a grid voltage range corresponding to a desired portion of the grid voltage-anode current characteristic of said discharge tubes, and means for simultaneously supplying currents from the anodes of said discharge tubes to said switching mechanism thereby to automatically interrupt the supply of energy to said X-ray tube at a predetermined value of the combined anode currents.

References Cited in the file of this patent UNITED STATES PATENTS 2,486,866 Morgan Nov. 1, 1949 2,488,315 Morgan Nov. 15, 1949 FOREIGN PATENTS 7 632,842 Great Britain Dec. 5, 1949 

